The fact that there was meaningful conversation between Moshe and Aharon while they were undoubtedly gripped by unspeakable pain over the shocking loss of Aharon's two sons speaks to the Grand Canyon between our spiritual plane and the lofty souls of Moshe and his brother. It therefore should not surprise us that Aharon actually found comfort in hearing (10:3), "This is what Hashem had said, 'b'krovai ekadesh... - Through those that are close to me I will become sanctified and I will be revered by the entire nation'". Yet the enigmatic substance of Moshe's remarks and the reference to an obscure pre-existing predictive teaching of Hashem has inspired many an interpretation.
According to Rashi, Moshe shares with Aharon that he had been taught that bringing Hashem's presence into a confined and defined physical space, an event so contrary to any natural event, demanded the sacrifice of our best. Perhaps these sacrifices were to forever teach the absolutely singular and imposing privilege of housing the Divine presence. With that in mind the enormous spirit of Aharon could indeed took solace and strength in the achievements and recognition of his two children and in the awareness that their sacrifice somehow was a necessary component of the realization of our national dream. Learning that this event, with all its mammoth pain and trauma, brought to life the words of Hashem, may have been uplifting as well, at least in retrospect.
Yet Ramban explains that there was no previous conversation to which Moshe seems to be alluding. With the support of numerous similar texts throughout Tanach, Ramban explains that Moshe says, "this is what Hashem wishes to communicate, that the manner in which He is approached is sanctified - i.e. determined solely by Hashem". Referencing the rulings that restrained Jews from ascending Har Sinai, all of whom were apparently drawn to revel in the greater sanctity of the closeness of Hashem, Ramban accords the highest motives to Nodav and Avihu. More importantly, we now understand that the emphasis on the boundaries of Har Sinai and its repeated teaching were all to curb and direct the powerful drive within an inspired person to gain closeness to Hashem. The practical realization of that lofty drive, when not part of Hashem's protocols and design, is censured, first in preparation for Sinai and now while initiating the service of the Mishkan.
Now let's paint the picture of our parsha according to Ramban. The moment of Hashem coming to dwell with His children had come. It had been anticipated by the collection of materials several months earlier and had been on hold mysteriously since Kislev. It was the climax of a seven day communal roller coaster of building the Mishkan, bringing scores of sacrifices, initiating the kohanim into service, waiting for the physical signs of Hashem's shechina, all to crash into the emptiness of the daily disappointing dismantling of the Mishkan. Most importantly, the forgiveness of the eigel sin, anticipated for over a half year - done!
The joy of that moment was unprecedented. Where else do we see the Jews dancing and prostrating all at once? Religious fervor and meaningfulness was at an apex. Nodov and Avihu "needed" to express their religious devotion in some fashion that speaks to their souls in a way that had not happened before, but was not so different at all - the same fire and the same pans and the same ketores. However, Hashem did not ask for this one.
At this moment we learned the lessons of the Sinai boundaries again and one that we would have to relearn over the centuries so many times: "bikrovai akodesh" - Hashem will be sanctified only in the manner that He ordains. Evidently, truly meaningful expressions of our innate spirituality all have to be found in our generous and existing corpus that relays to us Hashem's will.
The rabbinic explanations (see Kli Yakar who collects them) of the sin of Nodov and Avihu are many and varied including paskening in front of Moshe, drinking wine before the service, being unmarried and undesirous of children, and simply wanting to take over the reins of the generation. It is conceivable to me that these are all metaphors directing us to establish the tests of appropriate religious innovation.
Indeed, it is the converse of all these explanations of their sin - i.e. genuine concern and faith in seasoned and faithful leadership, responsibility to future generations, deeply anchored and cogently reasoned spiritual quests, and the direction of the Moshe Rabbeinu of the generation - that have enriched our legacy with chasidus, Bais Yaakov, mussar, religious Zionism and so many magnificent minhagim.